**4. Composition**

Minerals and rocks that make up the Earth's crust are the results of geological activity, density and tectonic plate movement. Minerals have definite chemical composition, whereas rocks are made up of minerals and have no specific chemical composition.

*Earth Crust*

theory is also unlikely as well.

**2.2 Origin of the oceanic crust**

**2.3 Origin of the continental crust**

crystallisation to take place to produce a granite crust [1].

under mountain ranges and thinnest under mid-ocean ridges [6].

of basalt, could have been formed by a basalt asteroid that impacted the Earth. However, from the observations of the moon, basalts found in lunar maria were not due to an asteroid collision. Furthermore, the number of basalts produced from an impact event was too insignificant to form crusts [1]. In addition, a majority of the impact events on Earth happened after oceanic crusts were formed. Therefore, this

The terrestrial model is the most likely explanation on the formation of the Earth's crust. This model explains that the crustal origin of the Earth was due to its internal processes. After the late accretion of the Earth, heat retained by the Earth resulted in the complete melting of the upper mantle, which formed a magma ocean that covered the surface of the Earth. As the Earth cooled, the magma ocean crystallised to form a widespread crust [1]. Another possible explanation was that the melted upper mantle rose up the surface to form a crust. The terrestrial model is the most likely explanation, as the magma ocean could explain some properties of the Earth's crust. The uniform composition of the crust could be formed by a homogeneous magma ocean. The layered composition of Earth's crust may be due to the cooling of magma oceans over time. Thus, the terrestrial model most likely explains the formation of the Earth's crust.

The oceanic crust was formed about 4.5 billion years ago, earlier than the first appearance of the continental crust, and it was first generated along the ocean ridges. The early oceanic crust differs from the present oceanic crust, in terms of its formation speed and thickness. The early crust was likely to be 20 km thick due to the high temperatures of the upper mantle. The higher mantle temperature caused a greater amount of melting in the upper mantle, resulting in more magma released to the surface to form thicker crusts [3]. The formation speed of the early oceanic crust was also likely to be faster than current speeds, due to the higher recycling rates caused by higher upper mantle temperatures [1]. The early oceanic crust is likely to be basalts in composition, and this could have resulted in the first plate tectonic activity. The basalt crust is denser than the molten mantle, so the basalt crust could have subsided into the upper mantle, leading to the recycling of crusts [3].

The oldest continental crust appeared about 4 billion years ago; however, granite continental crust only appeared about 3 billion years ago. There is no other planet in the solar system that has a continental crust except our Earth, mainly because it requires the presence of water on a planet and the subduction of crusts [4]. The seawater cools the hot mantle at the subduction zones, and it allows fractional

The Earth has a thin silicate crust, which makes up 1% of the Earth's volume [5]. It is the uppermost top component of the lithosphere and floats on top of the upper mantle [6]. The crust plus the upper mantle is separated by the Mohorovicic discontinuity—a seismic and compositional boundary [6]. The crust varies in thickness as controlled by the law of isostasy according to Airy's model—the crust responds to topographical changes (loads or unloads) by changing its thickness as compensation, thus tending towards isostatic equilibrium [7]. The crust is thickest

**2**

**3. Structure**

The three main kinds of rocks: igneous, sedimentary and metamorphic. Igneous rocks are formed by crystallisation of magma or lava. Sedimentary rocks are formed from lithification. Metamorphic rocks are formed from igneous and sedimentary rocks that undergone high temperatures and pressures, stress and fluid activity. The rock cycle ensures that these rocks are constantly replenished and recycled on the Earth's crust. Amongst the rocks, igneous rocks and metamorphic rocks make up 95% of the rocks [17], with granite and basalt having the largest compositions amongst the igneous rocks.

There are more than 3000 known minerals. Amongst them, only about 20 are common, and eight of these constitute 99% of the minerals in the crust. They are all silicates and are also called rock-forming minerals. Amongst the silicates, feldspars are the most abundant with plagioclase being the largest portion [18]. Minerals are formed by crystallisation through cooling of magma or lava and liquids. Another process is the evaporation of the liquid containing minerals, which result in the precipitation of material in the form of mineral veins.

### **4.1 Chemical composition**

Elements are the building blocks of minerals. Oxygen and silicon are the most common elements in the crust (**Figure 1**). The Earth's crust consists of both oceanic crust and continental crust. Despite silicates being the most abundant minerals in both crusts, there are still some differences in their characteristics (**Table 1**). Thus, deriving the average compositions of each respective type of crust is essential and critical to investigate the continents and the Earth as such knowledge provides insights into the origin and characteristics of the crusts.

There are three primary methods used to study the Earth's composition: (1) studying and interpreting the seismic profile of both the core and mantle, (2) studying other planets and meteorites and comparing and inferring their composition and (3) using the pyrolite model [19]. Data from all complementary fields need to be integrated to allow a comprehensive and consistent understanding of the current dynamic structure and composition of the Earth.

**5**

*Introductory Chapter: Earth Crust - Origin, Structure, Composition and Evolution*

Based on seismic investigations, the structure of continental crust is defined to consist of the upper crust, middle crust and lower crustal layers [10, 20]. Each layer varies slightly in its composition. The bulk composition is made mostly of rocks with a composition similar to granite rocks, full of substances such as oxygen,

Similarly, oceanic crust is also layered, and each layer varies slightly in its composition [21]. In general, oceanic crust is basaltic and is rich in minerals and substances like silicon, oxygen and magnesium. To determine the chemical composition, it is important to look into mid-ocean ridge basalt (MORB). All the MORB reflects the mean composition of no or the zero-age ocean crust apart from back-arc basins [22].

The evolution of the crust would refer to the gradual development of the crust over time. Geomorphically significant evolution of the Earth's crust falls into two main categories, endogenic processes from forces that originate within the Earth and exogenic processes that are a result of forces from above or on the planet surface.

Continental crust transforms into oceanic crust in a cyclic and dynamic process [23]. Where the old crust is being destroyed at convergent boundaries, new crust is being created at divergent boundaries. When rifting first occurs at divergent boundaries, the crust-mantle system transforms due to the temperature, and a rift forms. Subduction of the low-velocity zone in the upper part of the crust is the main mechanism overlooking the beginning of crustal attenuation. Intruding magma, originating from the mantle under the rift, modifies the intermediate and lower crustal layers. As the process continues, a "pseudo-oceanic" crust forms, which has an intermediate chemical composition. Before the new oceanic crust is created, the intermediate crust disappears completely, and the underneath crustal layer is critically modified by bouts of magma from the mantle sources. New oceanic crust is then produced from the ridge and spreads out from the spreading centre towards the subduction zone where the crust is eventually destroyed. Components of the crust will return to the upper crust in different forms such as igneous intrusions and contribute to the formation of new continental crust [21].Depending on the type of plate boundary and the types of plates involved, the resultant processes and landforms formed differ.

The different phenomena that occur contribute to the evolution of the crust.

*DOI: http://dx.doi.org/10.5772/intechopen.88100*

**4.2 Continental crust**

*Difference in characteristics.*

**Table 1.**

aluminium and silicon.

**4.3 Oceanic crust**

**5. Evolution**

**5.1 Endogenous factors**

**Figure 1.** *Composition of elements of the Earth's crust.*

*Introductory Chapter: Earth Crust - Origin, Structure, Composition and Evolution DOI: http://dx.doi.org/10.5772/intechopen.88100*


#### **Table 1.**

*Earth Crust*

amongst the igneous rocks.

**4.1 Chemical composition**

precipitation of material in the form of mineral veins.

insights into the origin and characteristics of the crusts.

current dynamic structure and composition of the Earth.

The three main kinds of rocks: igneous, sedimentary and metamorphic. Igneous rocks are formed by crystallisation of magma or lava. Sedimentary rocks are formed from lithification. Metamorphic rocks are formed from igneous and sedimentary rocks that undergone high temperatures and pressures, stress and fluid activity. The rock cycle ensures that these rocks are constantly replenished and recycled on the Earth's crust. Amongst the rocks, igneous rocks and metamorphic rocks make up 95% of the rocks [17], with granite and basalt having the largest compositions

There are more than 3000 known minerals. Amongst them, only about 20 are common, and eight of these constitute 99% of the minerals in the crust. They are all silicates and are also called rock-forming minerals. Amongst the silicates, feldspars are the most abundant with plagioclase being the largest portion [18]. Minerals are formed by crystallisation through cooling of magma or lava and liquids. Another process is the evaporation of the liquid containing minerals, which result in the

Elements are the building blocks of minerals. Oxygen and silicon are the most common elements in the crust (**Figure 1**). The Earth's crust consists of both oceanic crust and continental crust. Despite silicates being the most abundant minerals in both crusts, there are still some differences in their characteristics (**Table 1**). Thus, deriving the average compositions of each respective type of crust is essential and critical to investigate the continents and the Earth as such knowledge provides

There are three primary methods used to study the Earth's composition: (1) studying and interpreting the seismic profile of both the core and mantle, (2) studying other planets and meteorites and comparing and inferring their composition and (3) using the pyrolite model [19]. Data from all complementary fields need to be integrated to allow a comprehensive and consistent understanding of the

**4**

**Figure 1.**

*Composition of elements of the Earth's crust.*

*Difference in characteristics.*

## **4.2 Continental crust**

Based on seismic investigations, the structure of continental crust is defined to consist of the upper crust, middle crust and lower crustal layers [10, 20]. Each layer varies slightly in its composition. The bulk composition is made mostly of rocks with a composition similar to granite rocks, full of substances such as oxygen, aluminium and silicon.

#### **4.3 Oceanic crust**

Similarly, oceanic crust is also layered, and each layer varies slightly in its composition [21]. In general, oceanic crust is basaltic and is rich in minerals and substances like silicon, oxygen and magnesium. To determine the chemical composition, it is important to look into mid-ocean ridge basalt (MORB). All the MORB reflects the mean composition of no or the zero-age ocean crust apart from back-arc basins [22].
